Method of determining the surface characteristics of workpieces



Nov. 29, 1938. 'TCRNEBOHM 2,138,411

METHOD OF DETERMINING THE SURFACE CHARACTERISTICS OF WORKPIECES Filed March 4, 1956 4 Sheets-Sheet 1 Hilding Valdemar Tdrnebohm p his ATTORNEY.

Nov. 29, 1938. wRNEBoHM 2,138,411

METHOD'OF DETERMINING THE SURFACE CHARACTERISTICS OF WORKPIECES Filed March 4, 1936 4 Sheets-Sheet 2 INVENTOR. Hilding Valdemar Ttirnebohm BY I his ORNEY.

Nov. 29, 1938, H. v. TORNEBOHM METHOD OF. DETERMINING THE SURFACE CHARACTERISTICS OF WORKPIECES Filed Mardh 4, 1936 4 Sheets-Sheet 3 INVENTOR. Hilding Valdemar Tbrnebohm h is ATTORNEY.

Nov. 29, 1938. H. v. TGRNEBOHM 2,138,411

METHOD OF DETERMINING THE SURFACE CHARACTERISTICS OF WORKPIECES Filed March 4, 1956 4 Sheets-Sheet 4 Fig.7

Figfl.

' INVENTOR Hilding Valdemar T'drnebohm aa m H IS- ATTORNEY Patented Nov. 29, 1938 UNITED STATES PATENT OFFICE METHOD OF DETERMINING THE. SURFACE CHARACTERISTICS OF WORKPIECES Sweden Application March 4, 1936, Serial No. 67,033 In Sweden March 6, 1935 8 Claims.

The mechanical working of work pieces whether done by plastic deformation or by removal of chips results in a layer on the surface of the work pieces havingspecial characteristics and physical properties distinct from the properties characteristic for the material in itself. More and more attention has been paid during recent times to the great importance of the surface properties. It has thus been found that bad surface properties increase the corrosive tendency and tendency to fracture and thereby weakens the structure. The surface properties are also of great importance in other directions for the technical function for which the different work pieces are intended. It has thus been found that bad surface propertieshave a detrimental influence on the resistance to wear and that it increases the friction etc. Finally it may be mentioned that the elastic deformations which may, for instance, arise in press fits differ considerably from the deformations calculated according to the methods used in calculating the strength of material. The difference between the real and the calculated values cannot be referred to anything else than that the surfaces'of the work piece have other strength properties than that of the homogeneous material.

It is apparent that it would be of great advantage to be able to ascertain the surface properties in a simple manner, and especially if it were possible to ascertain the relation between the strength properties of the surface layer compared with those of homogeneous material. The attempts which have hitherto been made to ascertain surface properties have consisted in registering the surface either by photographic or mechanical means and also after having obtained suiiicient enlargement by these means, analyzing and by some method fixing the thickness of the surface layer or its characteristics. Such methods have, however, not been successful depending partly on the fact that the methods have either been based on subjective conception or that the mechanical registration of the surface must be carried out with the aid of a contact point which is only capable of incompletely reproducing the shape of the true surface. Such methods have therefore as a rule only been applicable for very bad surfaces but not for fine surfaces 55 having bad surface properties than into a work piece having good surface properties. In a work piece having a very excellent surface, for instance a block gauge in which the surfaces are so good that two blocks stick together due to the cooperation of the molecules in both surfaces, known mathematical formulas (Hertz formula) for the approach of two bodies subjected to forces perpendicular to their contacting surfaces are applicable. If a contact point of steel which has been carefully ground and polished to a hemisphere having 2.5 mm. radius of curvature is applied to'a block gauge the approach between the contact point and the block gauge will be 0.00052 mm. for a'load of 300 grammes, 0.00116 mm. for a load of 1000 grammes, 0.00137 mm. for a load of 1300 grammes, etc. In practice it will be impossible to calculate the penetration with reference to the unloaded position 0. It will instead be necessary to calculate the penetration for a certain difference in load. Since measuring apparatuses usually have a certain initial load of about 300 grammes it will be possible to measure the approach between bodies arising when the load is increased from 300 to 1300 grammes. This value will clearly be 0.00137 mm.-0.00052 mm.= 0.00085 mm. Upon comparing this calculated value with that obtained in practice it will be found that the agreement is very good when the measurement is carried out on block gauges. If, on the other hand, the measurement is carried out on bodies having less perfect surfaces than the block gauge, it will be found that the approach between the contact point and the work piece when the load is increased from 300 to 1300 grammes will have values which are greater than the above mentioned. On polished work pieces values of 0.001, 0.00105 to 0.0011 mm. have been obtained. On work pieces of fine ground finish the surfaces of which are less perfect than those of polished pieces still higher values will be obtained and on pieces turned or planed to a smooth finish a value of 0.002 mm. or more will be obtained. The above mentioned observations made during practical measurements have given rise to the present invention for the purpose of finding thestrength properties of the surface layer. The theoretical premises are as follows: According to Hertz the approach between two bodies subjected to a force normal to their contacting surfaces is calculated according to the formula:

$= opt k 2)] In this formula 5 is the approach between the bodies.

Eg' 117. where E2 is the modulus of elasticity for the work piece. This formula is valid for perfect surfaces-such as the surfaces of block gauges.

If the surface is less perfect it will be seen that the only factor which will be modified is 62 which will of course have another value. The worse the surface is, the greater must 02 be, since 6 increases when the surface is worse. If it is assumed that 02 for the Working surface is greater than 82 for the perfect surface by a fac-" tor 9 then 0 will express the quality of the surface.

, When 0:1, the surface is ideal, that it is as good quality of the surface, the less will be 0.

as that of the block gauge. The quality of this surface can, if it is preferred, be expressed in percent and will then be 100%. The worse the The percent value will fall and the surface quality can be expressed in percent of that of the gauge block by figures such as 75%, 60%, 50%, 30%, 20%, etc. It is clear that this percent value directly expresses the strength .quality of the surface layer since Hertzs constant is a function of Poissons constant and; the modulus of elasticity of the material. Assumed that Poisson's constant does not changear in other words that it is the same for the surface as for the homogeneous material, then 0 will be an expression for the diminishing of the modulus of elasticity of the surface layer compared with the modulus of elasticity of the homogeneous material. It may thereforez-besaid that 0 is the relation factor between th modulus of elasticity of the surface layer and the modulus of elasticity of the homogeneous material. I

As above mentioned measurements cannot be made at a load=0. It is therefore necessary to calculate with at least two loads, for instance P2, P1. The measured value will therefore be 6 for P2 minus a for P1. The Hertz formula for measurement on a perfect surface will then be as follows:

By introducing instead of 92' as above mentioned the formula will equation also be used with advantage for the purpose.

elastic.

In this equation 62 is the constant for the homo-' geneous material, according to Hertz, 01 is the.

constant for the contact point, A is the quantity,

the value of wihch depends on the radii of curvature of the contact point and of the work piece.

Its value is calculated according to the formula:

B=%01[P. 1 l

In the above mentionedformula 6 1 is; the measured value obtained on the surface under consideration.

As above mentioned the practical measurement of surfaces according to the invention is carried out with the aid of a sensitive measuring instru-' ment which indicates the elastic deformation of the surface layer arising from the contact point if pressed against the surface at at least two different loads. It is clear that a measurement of this kind can be carried out with the aid of practically any one of the usual types of known measuring instruments such as optimeters, minimeters or the like. Measuring instruments working on the known interference method can It is, however, necessary to make some alterations on the instruments in order to enable measurements to be carried out at at least two different loads. It is of course. further assumed that the instrument is sufficiently sensitive to be able to register results of less than 0.001 mm.

It is of course also possible to provide the measuring instrument itself such as an ultraoptimeter or the like with a loading device actuated either electrically, for instance by means of an electromagnet, pneumatically, hydraulically or mechanically. It is also possible to provide means in direct connection with the measuring instrument whereby the required load can be obtained by the application of weights.

For measuring the surface qualities of the surfaces of holes or of other locations diflicult of access it is of course necessary to combine the measuring instrument with a suitably formed I finger or the like according to known constructive principles. When measuring shafts or similar work pieces the surfaces of which are easily accessible from two opposite sides, it is preferred to form the aggregate so that two measuring contacts are obtained. The accuracy of the measurement is increased in this manner, since the scale reading on the instrument is doubled.

The measuring method in question presupposes that the deformations obtained are substantially In other words the stresses on the surface and in the contact point must therefore be kept suficiently low, in order not to exceed the limit of elasticity. For this reason the radius ,of curvature of the contact point is chosen sufficiently great and unduly great loads are avoided. On the other hand it is of importance that the reading on the instrument should be as great as possible and for this reason it is suitable to choose the contact points and the loads with respect to the material which is to be tested. It has been found suitable when measuring hardened steel to use a contact point having 2.5 mm. radius of curvature and a maximum load of 1300 grammes. For measuring unhardened steel a contact point having greater radius of curvature, for instance 5 mm., radius can be used. When measuring the surfaces of iron casting the radius of curvature of the contact points should be still greater. For this reason the contact pointsshould be conveniently interchangeable. The suitable radius of the contact point can be ascertained by trial by inserting a contact point having a small radius of curvature and applying the largest load for which the instrument is designed. Upon removal of the load it is easy to see whether or not the limit of elasticity has been exceeded, since in this case the index of the measuring apparatus does not return to its original position.

Some forms of measuring apparatuses for carrying out the measurements according to the invention are shown in the accompanying drawings, in which,

Figure 1 shows a measuring apparatus in which the sensibility has been increased by a lever.

Figure 2 shows a measuring apparatus in which the load directly actuates the contact point.

Figure 3 shows an apparatus for carrying out measurements on internal surfaces.

Figures 4, 5 and 6 show a device in which the load is applied hydraulically.

Figure '7 shows an enlarged view in section of the application of a measuring point to a perfect surface, and (paragraph) Figure 8 is a similar view showing the kind of a surface the instrument is intended to be used with.

The measuring apparatus shown in Figure 1 comprises a base plate I to which a lever 2 is jointedly connected. In order to eliminate all play in the joint and inv order that the friction in the joint will always be exactly the same the joint is made of thin leaf spring 3, which is connected to the base plate on the one side and to the lever on the other side by means of blocks 4 and 5 screwed to the parts mentioned. The weight of the lever is balanced by means of a spring 6, the force of which can be regulated by means of a nut 1. A contact point 8 is provided in the base plate and a similar contact point 9 isprovided in the lever. The contact point 8 is fixed while the contact point 9 is provided with threads whereby it is made adjustable for measuring work of different thicknesses or of different diameters. The contact point 9 is locked in desired position by means of a nut l0. On a raised portion ll of the base plate, is fixed a' measuring instrument l2, having a scale l3. The contact point M of the measuring instrument abuts against the upper surface of the lever near the end of the lever. It is therefore possible to read from the measuring instrument the displacement of the end of the lever for different loads on the contact point 9. The lever 2 is pierced at I5 for the shaft l6 of a weight IT. The shaft I6 is provided with a cross pin 26, which when the weight I! is in action rests in a V- shaped groove I8 in the lever 2. The weight I! can be lifted free of the lever 2, when it is to be put out of action.

The measurement is carried out as follows: The work I9 upon which the measurements are to be made is introduced between the contact points 8 and 9, and the contact point 9 is adjusted so that the lever 2 assumes the correct position which in this case 'is the horizontal. The ratio of leverage, the pressure from the contact point M and the reaction from the spring 3 are chosen .or adjusted so that the resulting pressure at the points of contact 8 and 9 will be-the first measuring load, for instance 300 grammes. The weight I! is held in raised position so that it does not actuate the lever. After having read the scale of the measuring instrument or after having adjusted it to the reading 0, the weight i1 is applied to the lever 2 so that the pressure on the points of contact 8 and 9 is increased with for instance 1 kg. The pressure will then be 1300 grammes. The new reading is taken on the scale 13 of the measuring instrument and the quality of the surface can be read from a table with the aid of the values obtained from the measurement.

The measuring instrument is provided with contact po'ints20 and 2| for measuring internal surfaces. These contact points can be adjusted to difierent distances from each other by means of screws 22 and 23 for measuring surfaces having different diameters.

By using two oppositely positioned contact points'the resultsof the measurement have been made independent of the influence of the support. When measuring, for instance cylindrical, turned or ground work pieces the reading will be doubled and the sensitiveness of the apparatus will be increased by the use of two similar contact points.

The apparatus according to Figure 2 com prises a frame 21, which is provided with feet 28, suitably 3 in number, which are radially movable in grooves 29, and which can be lodged in position at the desired distance from the center of the apparatus by means of nuts 30. A measuring instrument 3| is fixed to the frame2l and has a contact point 32 which is movable in relation to the measuring apparatus and to which is fixed a member 33. The member 33 is therefore movable in a vertical direction together with the contact point 32. The upper part of the member 33 is provided with a disc 34 to which a weight 35 can be applied when carrying out the measurement. The disc 34 is provided with a centrally located projection 36 which fits a hole 3l inthe Weight 35 so that member 33 will.

always be centrally loaded when the weight is applied.

When carrying out the measurement the appawith the pressure of the spring in the measuring A reading is taken on the Incas-- uring instrument after which the weight 35 is instrument 3 I applied and a new reading is taken. The weight 35 directly actuates the contact point 32. The distance of the frame 21 from the work piece will remain unchanged so that the changes in the position of the contact point relative to the frame depends entirely upon the penetration into the work piece by the contact point caused by the action of the weight 35.

The apparatus according to Figure 3 is especially suitable for measuring internal surfaces. It comprises a frame Ml,v which is provided with a vertical guide M. A slide 42, to which is fixed a measuring instrument 43 is in a known manner vertically adjustable on the guide M. The work piece M, in this case a ball bearing outer ring is introduced betweerrtwo contact points one of which, 45, is fixed to the frame 30, and the other, 46, belongs to the measuring instrument 43. On a pin ll fixed to the slide 42 a spring holder 48 is pivotally mounted. A leaf spring 49 is fixed to the holder 48. The spring is provided at its free end with a U-shaped shoe 50, which engages a flange on the contact point 46. The spring holder 48 can be turned about the pin 41 by means of a lever 52. The slide 42 is provided with an adjustment screw 53 for limiting the movement of the lever 52. The screw 53 can be lodged in desired position by means of a nut 54.

When carrying out the'measurement, the work piece 44 is hung upon the contact point 45, after which the slide 42 is adjusted on the guide 4| so that the contact point 46 engages the outer suriace of the work piece with the required initial pressure of 300 grammes. The apparatus can be adjusted so that this pressure is obtained when the index of the instrument points to 0 on the scale. The lever 52 is turned to the left as far as the screw 53 allows. The spring 49 is hereby bent and the shoe presses against the flange 5| on the-contact point 46 with a pressure cor.- responding to the desirg load increase, for instance 1 kg. This pressure is determined by a suitable adjustment of the screw 53.

The value read from the scale of the instrument represents the sum of the compression of both the inner and the outer surfaces of the work piece. In a ball bearing ring, for instance, this outer surface is ground while the surface of the ball race is polished. The compression at the race cannot therefore be calculated by halving the reading obtained from the reading of the instrument. In order therefore to find the coinpression or the penetration into the race, it is necessary to carry out a new measurement on the outer surface of the work piece. These measurements can be suitably carried out with the aid of the measuring apparatus shown in Figure 2 after which the penetration into the race can be is also as in Figure 3 hung upon the lower contact point 45. Between the instrument 43 and the work piece 44 a hydraulic load device of the following kind is introduced.

A member having the shape of a rectangular frame 56 is fixed to the spindle 55 of the instrument. The upper contact point 51 is fixed to the frame 56. The frame 56 and the contact point 51 are therefore movable together with the spindle 55 of the instrument. Fixed to the instrument 43 and vertically adjustable therewith is a pressure box 58. A pressure chamber 59 is provided in the box 58, one side of the chamber being covered by a diaphragm 60. The pressure chamber 59 communicates with .a cylinder 62 through a channel 6|. In the cylinder 62 is a piston 63 the movement of which against the pressure of the spring 64 is controlled by pressure on the button 65. A channel 66 leads from the channel 6| to the cylinder 62 behind the piston In the channel 66 is a pressuregoverning valve comprising a ball 61, the pressure of which against the seat 68 is governed by a spring 69. The pressure of the spring can be adjustedlby means of the screw 10. channel (ii, the pressure chamber 59 and the channel 66 are filled with a suitable pressure fluid. If pressure is applied to the button 65, the pressure in the pressure chamber 59 is increased,

peaks of the machined surface.

' contained between the The cylinder 62, the

been completed, the piston 63 is returned to its original position under the influence of the spring 64 whereby the fluid behind the piston leaks out past the packing of the piston.

The measurement is carried out thereby that the measuring instrument is first. adjusted to the proper height in the manner before described. For a certain reading on the scale, for example when the index points to 0 the pressure of the contact point 51 on the work piece will be the desired first measuring pressure, for instance 300 grammes. In this position there is suitably a small play between the upper end of contact point 51 and the diaphragm 60. The load on the contact point is then increased by pressing the button 65, whereby the pressure in the chamber 59 presses the diaphragm against the contact point 51 and actuates the same with the pressure determined by the governing valve.

Figure 7 shows on an exaggerated scale what happens, when a contact point II according to the invention engages. a perfect surface 12, for instance the surface of a block gauge. The dotted contour line 13 indicates the position of the contact point under the lighter load, while the full line H indicates the position and shape of the contact point under the second, heavier'load. It will be noted that the cont-act point itself becomes elastically deformed and has no longer the original spherical shape, which in the second position is indicated by the thinner full line 14. The impression made by the contact point is in dicated by the cross hatched area.

Figure 8 indicates what happens when the contact point is pressed against a machined surface. The surface is indicated by the wavy line. 15, the roughness of the surface being greatly exaggerated, in order to more clearly indicate thedifference between the conditions of Figure '7 and those of Figure 8. The position of the contact point under the very light load is here also indicated by the dotted line 13, and it will be seen that the contour of the contact point is tangent .to the When the load is increased, the contact point makes an impression in the surface of the workpiece and assumes the position indicated by the curve drawn with a full line 16. It will be noted that in assuming this position the peaks of the surface engaged by the contact point will become flattened out, this being indicated by the cross hatching 11. It will also be noted that the impression remains within, the surface'layer which may be defined as being the layer defined by the depth between the crests and the troughs of the surface; In Figure 8 the surface layer will thus be the layer horizontal dot and dash lines 11 and 19. J 2

It will be seen that the load required to obtain penetration to a certain determined depth is greater in the case illustrated in Figure 7 than in Figure 8. Conversely the penetration will be less in Figure 7 than in Figure 8 if the load is the same in both cases. The more perfect the surface, the greater is the resistance to penetration and it is this based.

The method according to the invention can be carried out in other ways without departing from the principle of the invention. It is thus for inquality upon which the invention is stance, possible to base the measurement on certain fixed depth for the penetration and measure the difference in load required for obtaining this penetration.

Having thus described my invention, I claim and desire to secure by Letters Patent:

1. The method of determining the surface characteristics of work pieces of metal or the like with respect to unevenness of the surface, wherein a contact point of known form and material quality is applied to the work piece, and the relationship between the penetration of the contact point into the work piece and the load on the contact point is determined for at least two different loads on the contact point of such magnitude that the permanent deformation caused by the contact point is limited to the surface layer of the work piece.

2. The method of determining the surface characteristics of work pieces of metal or the like with respect to unevenness of the surface, wherein a contact point of known form and material quality is applied to the work piece, and the relation'ship between the penetration of the contact point into the Work piece and the load on the contact point is determined for at least two different loads on the contact point of such magnitude that the elastic compression caused by the contact point is limited to the surface layer of the work piece, .that is to that part of the work piece defined by the depth between the crests and troughs of the surface.

3. The method of determining the surface characteristics of hardened steel work pieces with respect to unevenness of the surface which consists in applying a contact point having a spherical contact surface with about 2.5 mm. radius applied to the surface which is being investigated, at a pressure on the work piece of the magnitude of 1 to 2 kilograms or less and determining the relation between the penetration of the contact point into the work piece for at least two different loads on the contact point.

4. The method of determining the surface characteristics of work pieces of metal or the like with respect to unevenness of the surface wherein a contact point of known form and material quality is applied to the work piece and the difference between the penetration of the contact point into the work piece at at least two different previously determined loads is measured, the loads being of such magnitude that the permanent deformation caused by the contact point is limited to the surface layer of the work piece.

5. The method of determining the surface characteristics of work pieces of metal or the like with-respect to unevenness of the surface wherein a contact point of known form and material quality is applied to the work piece and the difference between the'penetration of the contact point into the work piece at at least two different previously determined loads is measured the loads on the contact point being of such magnitude that the elastic compression caused by the contact point is limited to the surface layer of the work piece, that is, to that part of the work piece designed by the depth between the crests and troughs of the surface.

6. The method of determining the surface characteristics of work pieces of metal or the like with respect to unevenness of the surface wherein a contact point of known form and material quality is applied to the work piece and the difference between two different measuring loads applied to the contact load is measured for a previously determined penetration of the contact point into the work piece, the loads on the contact point being of such magnitude that the permanent deformation caused by the contact point is limited to the surface layer of the work piece.

7. The method of determining the surface characteristics of work pieces of metal or the like with respect to unevenness of the surface wherein a contact point of known form and material quality is applied to the work piece and the difference between two different measuring loads applied to the contact point for a previously determined penetration of the contact point into the work piece is measured, the loads on the contact point being of such magnitude that the elastic compression caused by the contact point is limited to the surface layer of the work piece, that is to that part of the work piece determined by the depth between the crests and troughs of the surface.

8. The method of determining the surface characteristics of hardened steel work pieces with respect to unevenness of the surface which consists in applying a contact point having a spherical contact surface with about 2.5 mm. radius to the surface which is being investigated at a pressure on the work piece of the magnitude of one to two kilograms or less and measuring the difference between two measuring loads for a previously determined penetration of the contact point into the work.

HILDING VALDEMAR TRNEBOHM. 

